This invention relates to a device for amplifying and sampling multiplexed analog signals having a wide dynamic range of variations, i.e., whose level is liable to vary to a large extent.
The device according to the invention is particularly suitable for incorporation in a seismic data acquisition chain between a multiplexer and an analog-to-digital converter.
According to a known prior art process, the seismic signals issued from each geophone or geophone group are directly supplied to the inputs of an analog multiplexer after preliminary amplification in a preamplifier of fixed gain. The output of the multiplexer, where there are available the successive signals representing the samples, is connected to a single amplifier made up of a chain of serially-arranged amplifying elements, and with the output of each element being connected to the input of the next element. The gain of each amplification element is fixed and is preferably an integral power of the number two.
Since the difference in amplitude between two successive samples may be very high, a selection member is used to select the number of amplification units wherethrough each signal has to pass in order to reach an optimum level, and to switch the output of the amplification element where this signal is available onto the input of an analog-to-digital converter which is connected to a recording system. According to a first embodiment, described for example, the French Pat. No. 2,110,758, all the amplification elements have an identical gain. According to a second embodiment, the gains of the different amplification elements of the chain are different from one another and their values are selected from the successive powers of the number 2. An example of another arrangement, described in French Pat. No. 2,373,914, relates to an n-stage amplifier, wherein the gain value of any amplification stage is equal to the square of the gain value of the preceding amplification stage.
The amplifiers made up of a series of serially interconnected amplification elements suffer from a number of disadvantages:
They are relatively slow since the delays introduced by the different amplification elements wherethrough the signals have to pass are cumulative; PA1 the effects of the relative slowness of response of each amplification element, when operated with pulses, characterized by its voltage increase rate and, accumulated over all the chain of serially-connected elements, tends to reduce the overall speed of response of the amplifier; PA1 the time required for the gain selection member to select the output of the stage where the amplitude of the amplified sample is optimum, is proportional to the length of the amplification chain; PA1 when it is desired to shorten the amplification chain by increasing the gain of each stage, it becomes necessary to make use of an analog-to-digital converter capable of processing longer digital words in order to maintain the same accuracy; PA1 it may be further observed that the amplification elements are likely to become saturated by the passage of a sample of low amplitude followed by a sample of substantially higher amplitude.
In addition, a further disadvantage of the data acquisition chains having a head multiplexer, is that, in order to memorize a sample value during the period of gain selection, it becomes necessary to make use of a memory element, well known in the art as sample-and-hold unit, connected before the amplifier and, consequently, as a result of operating with signals in a wide dynamic range, the memory element must be capable of memorizing signals of high level as well as signals of low level. However, a known disadvantage of the sample-and-hold units which are available in practice, comes from the fact that they cannot successively memorize two samples without interaction of one of them on the other (i.e., diaphony or cross talk phenomenon), and that the memorized value of the sample amplitude, consequently, is not independent from the memorized value of the amplitude of the preceding sample. The errors due to the diaphony being applied to the input of an amplifier may be amplified with a gain equal to the maximum gain of the amplification chain. Moreover, sample-and-hold units are generally formed of a memorization capacitor and two buffer amplification elements which make the amplification chain longer. Accordingly, at least six amplification elements are required to form the amplifier.